This disclosure is generally directed to communication systems and more specifically to a frequency synthesizer using digital pre-distortion and method.
Wireless communication devices typically include a frequency synthesizer, which is often a critical component of many techniques used to modulate data onto an outgoing wireless signal. Conventional modulation techniques, such as Gaussian Frequency Shift Keyed (GFSK) modulation, are typically implemented using either I-Q up-conversion or by direct open-loop modulation of a voltage-controlled oscillator (VCO). The up-conversion approach is often susceptible to an undesired image signal and is typically costly in terms of analog area and power. The open-loop modulation approach is often prone to frequency drift in the voltage-controlled oscillator and to gain errors.
This disclosure provides a frequency synthesizer using digital pre-distortion and method.
In one aspect, a method includes identifying a setting for a capacitor bank associated with a voltage-controlled oscillator in a closed-loop phase-locked-loop. The setting represents a combination of one or more capacitors in the capacitor bank. The method also includes estimating a gain introduced by the closed-loop phase-locked-loop when the oscillator operates using the identified setting. The method further includes estimating a response of a loop filter in the phase-locked-loop and identifying one or more coefficients for a digital filter using the identified gain and the identified loop filter response. The digital filter is operable to filter an input signal. In addition, the method includes modulating the filtered input signal using the phase-locked-loop to produce an output signal.
One or more technical features may be present according to various embodiments of this disclosure. Particular embodiments of this disclosure may exhibit none, some, or all of the following features depending on the implementation. For example, in one embodiment, a frequency synthesizer using digital pre-distortion is provided. In particular, the frequency synthesizer uses closed-loop modulation of a phase-locked-loop to produce an output signal. The use of closed-loop modulation may introduce various irregularities in the output signal. As examples, the output signal may suffer from loop gain in the phase-locked-loop and from variations in a loop filter used in the phase-locked-loop.
To at least partially compensate for the various irregularities in the output signal, the frequency synthesizer also includes a digital pre-distortion filter. The pre-distortion filter distorts an input signal before it is modulated by the phase-locked-loop to produce the output signal. This distortion changes the input signal in such a way as to at least partially correct for the irregularities introduced by the phase-locked-loop. As a result, the output signal produced may have fewer or no irregularities. In a particular embodiment, the pre-distortion filter distorts the input signal such that the output signal is compliant with the Bluetooth standard.
This has outlined rather broadly several features of this disclosure so that those skilled in the art may better understand the DETAILED DESCRIPTION that follows. Additional features may be described later in this document. Those skilled in the art should appreciate that they may readily use the concepts and the specific embodiments disclosed as a basis for modifying or designing other structures for carrying out the same purposes of this disclosure. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.
Before undertaking the DETAILED DESCRIPTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document. The terms xe2x80x9cincludexe2x80x9d and xe2x80x9ccomprise,xe2x80x9d as well as derivatives thereof, mean inclusion without limitation. The term xe2x80x9corxe2x80x9d is inclusive, meaning and/or. The phrases xe2x80x9cassociated withxe2x80x9d and xe2x80x9cassociated therewith,xe2x80x9d as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like. The term xe2x80x9ccontrollerxe2x80x9d means any device, system, or part thereof that controls at least one operation. A controller may be implemented in hardware, firmware, or software, or a combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, and those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior as well as future uses of such defined words and phrases.